Conventionally, an X-ray imaging apparatus (X-ray phase imaging apparatus) for imaging an inside of an object utilizing a phase-contrast of an X-ray that passed through the object is known. Such an X-ray imaging apparatus is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2012-16370.
The X-ray imaging apparatus disclosed in the aforementioned Japanese Unexamined Patent Application Publication No. 2012-16370 includes an X-ray source, an X-ray image detector (detection unit) arranged in the irradiation direction of the X-ray source, and a plurality of gratings arranged between the X-ray source and the X-ray image detector. In the X-ray imaging apparatus disclosed in the aforementioned Japanese Unexamined Patent Application Publication No. 2012-16370, by interfering the X-ray irradiated from the X-ray source using a plural of gratings while moving any one of the plurality of gratings in the direction of the grating pitch, the intensity-modulated signal representing the intensity change of the X-ray detected by the X-ray image detector is obtained. In the X-ray imaging apparatus disclosed in the aforementioned Japanese Unexamined Patent Application Publication No. 2012-16370, it is configured to generate a phase differential image (image) in which the inside of the object is imaged based on the phase-contrast between the intensity-modulated signal when no object is placed between the plurality of gratings and the intensity-modulated signal when an object is placed therebetween. In the X-ray imaging apparatus of this kind, it is possible to image light element bodies or soft biological tissues which are less likely to absorb an X-ray by imaging the inside of the object using not the absorption amount of the X-ray but the phase-contrast of the X-ray.
However, in the X-ray imaging apparatus as disclosed in the aforementioned Japanese Unexamined Patent Application Publication No. 2012-16370, a positional displacement of the grating from a predetermined position may sometimes occur in accordance with fluctuations of the shape and size of the member (grating stage) that supports the grating due to the influence of, e.g., heat generated at around the apparatus. In this case, in cases where there is a time difference between acquisition of the intensity-modulated signal when no object is placed between a plurality of gratings and acquisition of the intensity-modulated signal when an object is placed between the plurality of gratings, positional displacements of the gratings due to heat influence occur at the position of the grating at the time of acquiring respective intensity-modulated signals. For this reason, the intensity-modulated signal for generating the phase differential image is entirely offset. This offset can be corrected during the generating process of the phase differential image. However, when the magnitude of this offset becomes, for example, near the half period of the phase of the intensity-modulated signal, a phase-contrast caused by the object is added in addition to the offset in the vicinity of the edge portion of the object. Phase folding (phase wrapping) occurs in which the phase-contrast between an intensity-modulated signal when no object is placed between the plurality of gratings and an intensity-modulated signal when the object is placed between the plurality of gratings is folded in the range of 2π. When this phase folding occurs, for example, even if the phase-contrast due to the object is (3/2)π, it cannot be distinguished from the case where the phase-contrast is −(½)π, which results in an erroneous measured value. As described above, in the X-ray imaging apparatus as disclosed in the aforementioned patent document, there is a problem that phase folding occurs in the phase differential image due to positional displacements of gratings caused by the influence of heat. In the present invention, the intensity-modulated signal is a concept representing an intensity change of an X-ray detected in an X-ray image detector when any one of a plurality of gratings is moved (stepped) by a distance corresponding to one cycle of a grating in a direction of a grating pitch with respect to an interference fringe formed by interfering an X-ray irradiated from an X-ray source by using a plurality of gratings.